1. Field of the Invention
The present invention relates to an optical material and, more specifically, to an optical material having an absorption characteristic or emission characteristic for a specific wavelength of light (specific wavelength light) peculiar to a metal ion.
2. Related Background Art
An example of conventionally known optical materials utilizing an absorption characteristic or emission characteristic for specific wavelength light expressed by a metal ion is one including a phosphonate monoester compound represented by the following formula (2) and a copper ion: 
where X1 and X2 indicate alkyl groups. As specific examples, commonly assigned Japanese Patent Application Laid-Open Nos. HEI 9-208775, 9-208863, 9-208918, and 9-211220 disclose phosphonate monoester copper compounds having X1 and X2 each of which has a carbon number of 1 to 20.
Such a phosphonate monoester copper compound has an absorption characteristic for near-infrared light peculiar to a copper ion and a transmission characteristic for visible light, thus being suitable as a material for an optical member having a near-infrared light absorptivity.
The inventors diligently studied the above-mentioned conventional phosphonate monoester copper compound from the viewpoints of transparency and optical characteristics when contained in a resin so as to form a resin-made optical member (e.g., resin sheet) which is excellent in versatility, ease of handling, and the like. As a result, there have been cases where, depending on resin bodies, a sufficient transparency is not obtained, so that they are hard to use as an optical member.
Further studies about causes thereof have revealed it desirable to further improve the compatibility with the phosphonate monoester copper compound and the resin, and that optical materials including other metal ions and phosphonate monoester compounds exhibit similar tendencies.
In view of such circumstances, it is an object of the present invention to provide an optical material which can improve the compatibility with a resin as compared with conventional ones, while having an excellent absorption characteristic or emission characteristic for specific wavelength light, thus making it possible to yield a resin-made optical member whose transparency is fully improved.
The inventors further conducted diligent studies in order to solve the problem mentioned above and, as a result, have attained a compound quite suitable for improving the compatibility, as a ligand for a metal ion such as copper ion, with a resin, thereby accomplishing the present invention. Namely, the optical material in accordance with the present invention contains a phosphonate monoester compound represented by the following formula (1): 
and a metal ion.
The metal ion is not limited in particular, but is preferably an ion of an alkali metal, alkaline-earth metal, transition metal, or rare-earth metal. Preferably, the optical material in accordance with the present invention contains an ion of a transition metal or rare-earth metal among others. In the present invention, xe2x80x9ctransition metalxe2x80x9d refers to metals having an atomic number of 21 (scandium) to 30 (zinc), 39 (yttrium) to 48 (cadmium), and 72 (hafnium) to 80 (mercury).
These metals (ions) express absorption characteristics or emission characteristics peculiar to atomic structures, so that the use of these metals yields optical materials having various characteristics. In particular, the transition metals and rare-earth metals can form an optical material excellent in functionality, since they express infrared light absorption characteristics assumed to be caused by electron transitions in d-orbit or f-orbit, or a visible light absorption or emission characteristic, whereby an optical material excellent in functionality can be formed.
Further, useful metals among these materials are sodium, potassium, magnesium, calcium, iron, manganese, nickel, cobalt, chromium, copper, neodymium, praseodymium, europium, thulium, erbium, terbium, dysprosium, samarium, lanthanum, gadolinium, holmium, and the like.
Among others, it will be particularly preferable for the optical material of the present invention if the metal ion is an ion of at least one metal of iron, manganese, nickel, cobalt, chromium, copper, neodymium, praseodymium, europium, thulium, erbium, terbium, dysprosium, samarium, lanthanum, gadolinium, and holmium.
In particular, copper can coordinate or combine with the phosphonate monoester compound, thereby favorably expressing a quite excellent infrared light absorption characteristic or visible light transmission characteristic. On the other hand, neodymium, praseodymium, europium, thulium, or erbium has a large, steep absorption wavelength peak, thus being excellent in wavelength component selectivity and tending to yield a higher emission efficiency for fluorescence and the like.
The optical member containing the phosphonate monoester compound represented by formula (1) and a copper ion, and the conventional optical material containing the phosphonate monoester compound represented by formula (2) and a copper ion were dissolved or dispersed into a monomer constituting the resin, and their optical spectra were measured. This has verified that the one in accordance with the present invention has an spectral characteristic superior to that of the conventional one.
Also proposed as other conventional optical materials are (1) one using a phosphate ester compound as a ligand for a metal ion (see, for example, commonly assigned International Publication WO9926952), (2) one using a phosphinate compound (see, for example, Japanese Patent Application Laid-Open No. 2000-98130), and (3) one using a phosphonate compound (see, for example, Japanese Patent Application Laid-Open No. 2000-7687).
The inventors also investigated and studied these optical materials from various viewpoints, and verified the superiority of the optical material in accordance with the present invention. Specifically, the optical material in accordance with the present invention exhibits an improved heat resistance as compared with the one using a phosphate ester compound in accordance with (1) and the one using a phosphinate compound in accordance with (2). Also, it has been found that the solubility or dissolution stability with respect to monomers constituting the resin drastically improves as compared with the one using a phosphonate compound in accordance with (3).
It will also be preferable if the phosphonate monoester represented by formula (1) and the copper ion are contained in a solvent or a resin. In this case, characteristics and properties corresponding to the solvent and resin used are imparted to the optical material and/or the optical member manufactured by using the same. Therefore, optical materials suitable for various uses can be obtained if the solvent and resin are selected appropriately. The present invention will be more fully understood from the detailed description given herein below and the accompanying drawings, which are given by way of illustration only and are not to be considered as limiting the present invention.
Further scope of applicability of the present invention will become apparent from the detailed description given herein after. However, it should be understood that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will be apparent to those skilled in the art from this detailed description.